1. Field of the Invention
This invention relates to a four-wheel steering system for a vehicle in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel.
2. Description of the Prior Art
There is disclosed, for example in U.S. Pat. No. 4,313,514, a four-wheel steering system for a vehicle in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel, the ratio .theta.R/.theta.F of the rear wheel turning angle .theta.R to the front wheel turning angle .theta.F (this ratio will hereinbelow be referred to as the "rear wheel steering ratio") being changed according to the vehicle speed and the front wheel turning ratio. Generally, the rear wheels are turned in the same direction as the front wheels (same phase) when the vehicle speed is relatively high and in the opposite direction to the front wheels (reverse phase) when the vehicle speed is lower than a predetermined value. By turning the front and rear wheels in the same phase, side slip of the wheels can be prevented during high speed travel, thereby improving running stability, and by turning the front and rear wheels in the reverse phase, the minimum turning radius of the vehicle can be reduced.
However, there is a problem in the four-wheel steering system that when the rear wheel steering ratio .theta.R/.theta.F is fixed irrespective of the road condition, skidding of the wheels can occur irrespective of the vehicle speed. For example, when the road is wet with rain and the friction coefficient of the road is lowered, the road gripping force of the wheels is lowered and skidding of the wheels is apt to occur. The road gripping force of the wheels can also be lowered when the road surface is rough and the vehicle body vibrates up and down.